The Rain-shadow Effect
Sheltered by mountains
by Philip Eden
"Rain shadow" is a phrase often used by meteorologists in their day-to-day work, and is occasionally mentioned by television weather forecasters. Some may regard it as a solecism, but it well describes the phenomenon whereby much less rain falls on the lee side of a mountain range compared with the windward side. Last week produced one of the best examples of the rain-shadow effect we have seen in the UK in recent years thanks to a persistent southwesterly airflow.
Things are very different on the leeward side of the mountains. The air-mass has now lost much of its moisture, and as the winds descend the lee slope the air becomes denser again, and therefore warmer. As it warms up its capacity to hold moisture increases again, thus it is no longer saturated. The mechanism which produced the persistent rain on the windward slope is now switched off, the rain stops, and the clouds dissipate.
Last week's rain shadow effect (see plot above) was very prominent in the shelter of the Scottish Highlands. During the week ending 0600 on Saturday (Jan 19) almost 100mm of rain was recorded in the Strath of Orchy in Argyllshire and an estimated 150mm fell in the upper parts of nearby Glen Strae and Glen Kinglass. By contrast no measurable rain at all fell at Kinloss in Morayshire, and only 2mm at Aberdeen. The effect was also well illustrated in northern England where Shap in Cumbria recorded 83mm of rain compared with 4mm at Newcastle, and also in Wales and the Midlands with 96mm at Capel Curig in Snowdonia but only 2mm at several sites in the Midlands.